Leg top corrosion/oxidation

Discussion in 'Stove Forum' started by Twoberth, Mar 14, 2019.

  1. Twoberth

    Twoberth United States Subscriber

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    Hi,
    When I obtain old stoves with the burner in reasonable condition, (indicating reasonable but not excessive use), it seems to me that the top of the legs on old stoves that use a trivet seem more corroded that ones without a trivet.

    Given that the leg tops interfere with the complete combustion of the paraffin vapour, does the trivet make this worse and does this incompletely combusted atmosphere cause more rapid corrosion?

    Or then again, am I imagining things?
     
  2. snwcmpr

    snwcmpr Subscriber

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    Are the metals different? Is there bi-metal corrosion?
    Could moisture accumulate after use?
     
  3. Lennart F Sweden

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    Steel legs on nearly every stove - it seemes corrosion is more common under cast iron trivets than the pressed steel ones.
     
  4. kerophile

    kerophile United Kingdom Subscriber

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    Hi, I believe that legs on the majority of classic brass stoves were "tinned" with either pure tin or a tin-lead alloy, using a hot-dipping process.

    The steel legs would have been thoroughly cleaned of all oxide and grease, fluxed, and then dipped into a pot of molten tin or solder. I have seen evidence on several stove legs that this was done, one end at a time, as there is a 'tide-mark' half-way up each leg.

    The aim would have been to get a thin uniform coating on each leg to ensure corrosion protection, and for fixed-leg stoves, subsequent successful soldering to the tank.

    Once in use the protective tin coating of the legs was prone to damage and partial removal. This is particularly true in those parts of the legs which get very hot in use, due to contact with the flame.

    When the underlying steel is exposed, corrosion will occur much faster than on parts still having a protective tin coating.
    This is the reason those areas at the top of the legs (which also happen to contact the trivet) corrode and thin.

    Simples.

    Retinning Stove Legs

    Best Regards,
    Kerophile.
     
  5. Twoberth

    Twoberth United States Subscriber

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    Yes, I would agree with that. Corrosion under cast iron seems worse than pressed steel.
     
  6. Twoberth

    Twoberth United States Subscriber

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    @kerophile Clearly the leg tops corrode more because they get hottest, but why would they corrode faster under a trivet?
     
  7. kerophile

    kerophile United Kingdom Subscriber

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    Hi @Twoberth . Cast-iron trivets were a feature of most of the early classic brass stoves.
    Later, trivets were assembled from pressed steel elements.
    Finally, one-piece pressings were adopted for the vast majority of brass stoves, with cast-iron trivets reserved for only a few of the biggest stoves.

    So, the oldest stoves, subject to the longest time for corrosion to occur will likely have had cast-iron trivets. Those stoves with one-piece pressed steel trivets, are likely much younger, and will have had less time for corrosion of the legs.

    Best Regards,
    Kerophile.
     
  8. Radler Switzerland

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    Full agree with @kerophile.
    There is a second point. Corrosion is a chemical process and, as most of them, is faster at high temperature. Often 10°C more means the same process in half the time. The ends of the supports may easily reach 300°C and more. Melting point of tin is 232°C.
    The flue gas stream consists mainly of nitrogen (neutral), carbon dioxide (CO2) and water (H2O). As long as the iron is in the stream of this gases, very little corrosion occurs because of absence of oxygen. But every turbulence brings oxygen from the surrounding air to the hot surface of the iron.
    A trivet does always cause such turbulences, the angular shape of cast iron trivets maybe more than others.

    Regards

    Radler
     
  9. Simes

    Simes Subscriber

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    I do quite like your conclusion @Radler, it does make sense.

    Does that suggest that most old collapsables legs are potentially not original if in good condition?
     
  10. Ed Winskill

    Ed Winskill United States Subscriber

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    Or not used all that much; a common situation.
     
  11. Twoberth

    Twoberth United States Subscriber

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  12. kerophile

    kerophile United Kingdom Subscriber

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    The additional factor that I should have mentioned is that each heating and cooling cycle experienced by the rusting legs will detach rust which formed earlier. This will accelerate overall metal removal:



    Best Regards,
    Kerophile.
     
  13. OMC

    OMC Subscriber

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    re OP: "...old stoves…"
    I will discuss *old as in c1900-1915. For example earlier/earliest 3 leggers had a noticeably tighter bend at the top of the legs. No pics. Do examples you refer to have the old tight bend?

    and "...it seems to me that the top of the legs on old stoves that use a trivet seem more corroded that ones without a trivet."

    I'm not sure about that one as described :-k.
    Myself and others however have researched *old 3 leggers and trivet related corrosion hadn't come up.

    There was a pot support issue, and by chance if that is what you refer to, here goes
    [​IMG]
    credit igh137 for images

    early cast trivet above has 3 tangs that extend nearer the burner.
    IF stove were just slightly earlier, the stove's pot supports would also extend nearer the burner. <-- pot supports too close to burner was a problem.

    Re OP: "...corrosion / oxidation.." aka rust :content: .
    1. super heat exposes, bare steel (**more heat = more exposed steel, more oxidation/damage).
    2. Stove cools, moisture in atmosphere oxidizes on surface (surface rust).
    3. super-heat, +1 with kerophile's video, rust on surface flakes off (daily use repeats 1-3 hundreds of times).
    So
    legs on MOST *old stoves, that extend nearer the burner, had problem , get super-heated … shed their outer layer, rust, repeat and became thinner and weak.
    bentLegs.JPG

    By say c1907 Primus knew they had a problem, so they...
    ...shortened the pot supports. That exposes supports to less heat (**less heat = less exposed steel).
    ... they may have done more than that, quality of steel used, the tinning kerophile describes, ...
    Whatever all they did: problem solved.
    The pot support damage common on earlier 3 leggers does not occur on later models w/pot supports away from burner. Oh, later models, as in c1911 and later, it's all relative :content:.
    This may or may not be the issue you observe but re OP: "Leg top corrosion" (cast trivets) that was an issue, jus sayin'. thx omc
     
    Last edited: Mar 15, 2019
  14. Radler Switzerland

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    The picture with the yellow background shows quite well, the heath is not the problem.
    The ends of the supports, certainly the hottest part, are less corroded because they were in the flue-gas stream. Too little oxygen is available there for oxidation.
    Most corrosion is done in the middle of the horizontal part, were the flue-gas gets mixed with air.

    Regards
    Radler
     
    Last edited: Mar 15, 2019
  15. OMC

    OMC Subscriber

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    For that example, ya got me there. :-k

    I would caution that conclusion with a broad brush.
    The entirety of the leg is weaker and IIRC, there are ample examples with the tips are thinnest. and
    "c1907 ... Primus knew they had a problem, so they...
    ...shortened the pot supports. …
    Whatever all they did: problem solved
    ."

    well received, thx omc
     
  16. Twoberth

    Twoberth United States Subscriber

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    Interesting comments from all, and thanks for the input.

    @OMC Regarding oxidation being fastest nearest the flame. This is what you would expect but it isn't that simple as your own photo above shows. In your photo, and on several of my old stoves, the corrosion/erosion/oxidation is most severe in the middle of the horizonal top, i.e. where the clips on the trivet would be.

    DSC07627.JPG DSC07628.JPG DSC07629.JPG

    The corrosion eventually causes the middle to thin so much the ends break off

    DSC07631.JPG DSC07631.JPG DSC07632.JPG

    They don't corrode outwards (backwards) from the end, they actually corrode in the middle and then break.

    DSC07633.JPG DSC07634.JPG

    I suspect @Radler is right about turbulence at the trivet ends/clips and entrained air causing rapid localised corrosion of the middle of the leg tops (and for that matter often causing corrosion of the trivet clips).

    As Georges says, 'Aren't stoves fascinating'.
     

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  17. Twoberth

    Twoberth United States Subscriber

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    @Radler You must have been typing while I was uploading photos. Same message though.
     
  18. Simes

    Simes Subscriber

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    Is it possibly just a condensation issue on cooling down?

    Especially where the trivet sits will trap water there accelerating the process I would have thought.

    One of the principle.by products of the combustion process is water, also pots get quite wet on the base during the initial heating period before everything is warmed up.
     
  19. Twoberth

    Twoberth United States Subscriber

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    @Simes I don't think it is a condensation issue, although there is no doubt that there is a lot of water vapour produced by the combustion process

    As you say, the pots get wet during the initial heating period, but when everything gets hot - everything dries out. After cooking the leg tops stay hot for several minutes (as my fingers will attest) so I don't think localised condensation is a contributor.

    However, whatever causes it, localised leg top corrosion is real and the trivet doesn't help.
    In fact, at the risk of being shot down in flames, I would suggest that leg tops last longer without trivets than with. I also suspect that pointed or rounded leg ends which will promote smooth gas flow over them will last longer than square ended legs.

    Anyone want to do the thirty year experiment?
     
  20. Simes

    Simes Subscriber

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    @Twoberth, pure speculation on my part

    I'll pop out and buy a new stove with trivet tomorrow and promise to use it every day, twice a day and report back my results in 2049. :)